Method for reducing uranium trioxide

ABSTRACT

Uranium trioxide Is reduced to uranium dioxide using microwave radiation or radiofrequency radiation directed in such a way that the radiation encounters an Interface between uranium trioxide and the uranium-containing reduction product without first having passed through that product. By this method, and also using a reducing gas, it is possible to obtain UO 2  with an O:U ratio less than 2.04:1.

FIELD OF THE INVENTION

The present invention provides a method and apparatus for reducing uranium trioxide, UO₃, to a lower oxide of uranium, preferably uranium dioxide, UO₂.

BACKGROUND OF THE INVENTION

Uranium dioxide is used to produce ceramic UO₂ pellets for nuclear power plants, or as a source for production of uranium tetrafluoride, UF₄. The UF₄ is then further used to produce metallic uranium, or is converted to uranium hexafluoride, UF₆, by fluorination. Conventional methods for producing UO₂ typically yield a product with an oxygen to uranium ratio of 2.04 or greater. It is important that the uranium dioxide feedstock for fluorination have an oxygen to uranium ratio close to 2.00. Uranium dioxide feedstocks in which this ratio is significantly higher than 2.00 cause formation of uranyl fluoride, UO₂F₂, which contaminates the UF₄ product. This reduces yields and increases the cost of fluorination.

One method for preparing uranium dioxide is to reduce pulverized uranium trioxide powder in a hydrogen atmosphere, usually in a conventionally heated fluidized bed, at a temperature of about 7000° C. The chemical conversion is represented by equation (1)

UO₃+H₂→UO₂+H₂O  (1).

The UO₂ obtained under these conditions normally has a minimum O:U ratio of 2.04. To reduce this ratio further, two approaches are known: 1) the particle size of the UO₃ starting material is reduced to the micrometer range; or 2) a higher temperature is used. Neither of these solutions is particularly satisfactory. To obtain a sufficiently small UO₃ particle size, special precipitation techniques may be required. Such techniques are time-consuming, often difficult to carry out, and expensive. Option 2), increasing the temperature of a fluidized bed reactor, requires a considerable increase in energy consumption. Heat transfer in such reactors is highly inefficient due to low thermal conductivity of the bed, and significant heat is lost with the flow of fluidizing gas.

The fluidized bed method suffers from other drawbacks. High flow rates of carrier gas and hydrogen are required to support the fluidization, with the result that hydrogen consumption is approximately 170% to 190% of the stoichiometric amount indicated by equation (1). The UO₃ starting material must be pulverized prior to feeding into the reactor. The process is highly sensitive to particle size distribution. Furthermore, high maintenance costs are incurred because heating elements must be periodically replaced, requiring shutting down of the reactor.

It has been proposed to produce sinterable UO₂ by the use of microwave heating [Canadian patent No. 1,197,069; Thornton, Thomas A.; Holaday, Veldon D., Jr., which is incorporated herein by reference]. Thornton et al propose a process that involves the absorption of microwave radiation by uranyl nitrate hexahydrate (UNH), ammonium diuranate (ADU) or ammonium uranyl carbonate (AUC). These uranium salts are decomposed, preferably in an oxidizing atmosphere, at elevated temperatures, to yield an intermediate product which may have a uranium oxide stoichiometric range of from UO₃ to U₃O₈. The intermediate product is further heated in a microwave furnace, in a reducing atmosphere, to reduce it to sinterable uranium dioxide powder.

Thornton et al, in U.S. Pat. No. 4,389,355, which is incorporated herein by reference, have proposed a process for preparing nuclear fuel pellets that involves sintering UO₂ powder and an organic binder in a microwave induction furnace in a reducing atmosphere. Sintered compacts are cooled under reducing atmospheric conditions and then ground to the desired finished uranium dioxide pellet product. Scrap uranium dioxide powder and rejected pellets are recycled to a microwave induction furnace where they are heated in an oxidizing atmosphere to convert UO₂ to U₃O₈. The U₃O₈ is then blended with UO₂ and organic binder at the beginning step of the nuclear fuel pellet preparation process.

Ford and Pei, in the Journal of Microwave Power, 2—2, 1967, pages 61 to 64, the disclosure of which is incorporated herein by reference, propose heating various materials, including uranium dioxide, by microwave radiation.

Paul Haas discusses heating uranium oxides in a microwave oven; see page 873 of the American Ceramic Society Bulletin, Volume 58, No. 9, (1979) the disclosure of which is incorporated herein by reference. Haas found that UO₂ and U₃O₈ samples heated strongly under microwave irradiation. Dry UO₃ samples did not show any significant heating when exposed in a microwave oven. However, in tests on microwave drying of samples of hydrated UO₃ gel spheres, hot spots were observed to develop. Haas suggested that the hydrated gel first underwent small amounts of reduction from traces of NH₃ and organic materials in the gel. Once overheating started, hexavalent uranium was converted to U₃O₈, which absorbs microwave energy.

Use of microwave energy to heat oxides of uranium Is also discussed by Van Loock and Tollenaere, Sprechsaal, Vol 122, No. 12, 1989, pages 1157-1159 and by Sturcken and McCurry, in Ceramic Transactions 1991, Volume 21, pages 117-123, the disclosures of which are incorporated herein by reference.

A problem encountered when microwave energy is used to heat uranium oxides to convert them to UO₂ is that the microwave energy is absorbed and attenuated in the outer layer of the irradiated uranium oxides, where the UO₂ product is first formed, and does not penetrate deeply into the material. The interior of the material is screened by the outer layer of UO₂, so that the microwave energy is absorbed in the outer layer and does not penetrate to the interior. This leads to overheating of the outer layer (thermal runaway), resulting in non-uniform heating, and a cool reactor core. Ford and Pei encountered non-uniform heating of uranium dioxide which, they say, was very unsatisfactory and caused a suspension of experiments. It may be possible to reduce this screening effect and to avoid non-uniform heating by sophisticated powder mixing techniques, but these techniques are not practical on a large industrial scale.

Thus, the idea of direct microwave heating of uranium oxides with microwaves in a single mode resonant cavity or multimode oven applicators has been demonstrated with small loads, less than about 50 grams, by the several above-mentioned workers, whose publications are incorporated herein by reference. However, this technique is not feasible for larger scale applications, where the dimensions of processing material load are much greater, so that the desired length of the path of the radiation in the material is much greater than the penetration depth of the microwave radiation. The penetration depth of microwave radiation into UO₂ is in the millimeter or centimeter range. Due to the attenuation of the microwave power in the outer layers of the reduced UO₂ product, the inner part of the processing material load will not be irradiated, and heating of the inert part occurs by thermal conductivity and load mixing only.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides a method of reducing UO₃ which comprises subjecting UO₃ to heat generated by microwave or radiofrequency radiation, wherein the radiation is supplied from a direction in which it encounters an interface between UO₃ and another oxide of uranium, without having first passed through the said other oxide of uranium.

Thermal decomposition of UO₃ to U₃O₈ takes place at high temperature, say about 900° C., and can be carried out with microwave or radiofrequency radiation. Preferably, however, the reaction is carried out in the presence of a reducing gas and the product of reduction is UO₂, particularly UO₂ having an O:U ratio that is less than 2.04:1.

In another aspect the invention provides an apparatus for use in the above method, which apparatus comprises a container for containing UO₃ and the said other oxide of uranium, and a source of microwave radiation or radiofrequency radiation that directs the radiation towards the interface between the UO₃ and the said other oxide of uranium in such a manner that, in operation of the apparatus, the radiation encounters the interface without having first passed through the said other oxide of uranium. In use, the apparatus is preferably connected to a source of reducing gas, which gas passes through the reaction mixture. The gas preferably passes through the reaction zone and then into the UO₃, thereby assisting in heating the UO₃ by convection.

The advantages of using microwave heating are manifold. Microwave energy is absorbed directly by the material being processed, provided that the material has a sufficient “dielectric loss factor” (the dielectric loss factor reflects the extent to which a material converts microwave energy to heat). The microwave energy can be transported from a generator to a load using waveguides, and the microwave generating device (magnetron) can be quickly and easily replaced when necessary, without shutting down the entire process.

When a material has sufficient dielectric loss factor most of the incident microwave power is absorbed within the material, and heat is liberated in sufficient quantity to initiate the reaction. The rate of heat liberation is proportional to the product of the dielectric loss factor and the density of microwave power. The value of the dielectric loss factor (the imaginary part of the dielectric constant) for uranium oxides is a function of their composition and the frequency of the electromagnetic (microwave or radiofrequency) field; it also changes with temperature.

UO₂ has high dielectric loss factor and therefore is readily heated by microwave or radiofrequency radiation. Pure dry UO₃ is not heated by microwave or radiofrequency radiation. At the interface between UO₃ and UO₂ the radiation encounters and heats UO₂. Heat is then transferred by conduction, and also by convection with the reducing gas, from the heated UO₂ to the UO₃. The UO₃ decomposes, under the influence of the heat and the reducing gas, to form UO₂. At the UO₂/UO₃ interface, therefore, there is formed a reaction zone that propagates by advancing into the mass of UO₃ as the reaction proceeds. Problems caused by UO₂ shielding material from radiation are therefore reduced or avoided.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows, by way of example and in schematic form, one embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As stated above, pure dry UO₃ is not heated by the radiation. Hence, it is convenient to direct the radiation through the UO₃ to the UO₂/UO₃ interface.

It will be appreciated that heat is required to initiate the decomposition of UO₃ to microwave absorbing species. The heat can be supplied by any suitable means. It is convenient to use an initiator, which may be any material, including non-uranium material, which absorbs the microwave energy and can transfer the liberated heat to the UO₃ feed material. It is preferred to use a uranium oxide initiator, however, as a non-uranium initiator may contaminate the UO₂ product. The initial high temperature in a layer of UO₃, required for initiation, can also be created by using means other than microwave technology, such as conventional heating of a localized volume of the UO₃ bed, utilization of an electric current through a conducting medium, electric arc, or any other means to cause decomposition or reduction in a localized area of the UO₃ load so that an interface is created between UO₃ and a microwave absorbing material.

If a microwave-absorbing material is to be used as initiator, it is desirable that the length of the path of the microwave radiation in the layer of absorbing material shall be equal to the penetration depth of the microwave radiation in the material, or greater. Microwave energy is exponentially attenuated in an absorbing material. The heat released due to the microwave absorption at a depth x from the surface may be presented as P(x)=P(0) exp(−x/D), where P(0) is the heat released near the surface and D is the penetration depth of the microwaves in the material under consideration. This formula shows that the released heat decreases 2.7 times at a distance D, which means that 63% of the microwave energy passing through the material is absorbed within the layer of penetration depth. When the thickness is much greater than the penetration depth, most of the microwave energy passing through the material is absorbed, releasing heat. This is the case, for instance, when the UO₃ reduction process has been proceeding for a while and the reaction zone has propagated significant distance, in particular in the continuous mode. If the thickness of the absorbing layer (initiator) is too small, only a fraction of the microwave power is absorbed, and this may be not sufficient to initiate the reduction of UO₃.

Oxides of uranium are sometimes classified as stoichiometric or non-stoichiometric. Stoichiometric oxides of uranium are characterized by the existence of definite crystalline phases that may usually be identified using diffraction techniques (X-Ray, electron or neutron diffraction, EXAFS, etc.). Such phases have a composition expressed in simple ratio numbers, e.g. UO₂, U₃O₈, U₄O₉, UO₃. Depending of the material history and environment, the ordered crystal structure of a single phase of a uranium oxide may have defects such as the absence of some oxygen atoms in the lattice or an excess of them (interstitial atoms), and oxides with such defects are known as non-stoichiometric oxide. Examples of non-stoichiometric compounds include UO_(2+x), U₃O_(8−x) and U₄O_(9−y). Dielectric characteristics of non-stoichiometric uranium oxides strongly depend on the concentration of such defects (i.e., on the composition of the material). Practically all of the uranium oxides, excluding UO₃, absorb microwave energy. The invention can use stoichiometric or non-stoichiometric oxides as initiators, and oxides that may be formed as intermediates in the reaction may be stoichiometric or non-stoichiometric.

The method can be carried out in batch fashion. In one embodiment of a batch process, a reactor is loaded with UO₃ and a small quantity of UO₂ to serve as initiator, so that there is formed a UO₂/UO₃ interface. The thickness of the UO₂ layer is preferably of the order of the penetration depth of the microwave radiation employed, or greater. In the case of 2450 MHz or 915 MHz radiation, it is preferably equal to or greater than about 3 mm. It is desirable that the thickness of the layer of initiator (UO₂ or U₃O₈ or U₄O₉ or their mixture or any other non-stoichiometric uranium oxide absorbing the microwave power or any other material absorbing the microwave power) be of the order of, or greater than, the penetration depth of the microwave radiation used in the process. As the reaction proceeds a reaction zone forms at the interface and advances through the reactor as UO₃ is converted to UO₂. The direction of flow of the reducing gas is preferably countercurrent to the direction of advance of the reaction zone.

Alternatively, the method can be carried out in continuous fashion. UO₃ is continuously fed into the reactor and formed UO₂ is continuously withdrawn from the reactor. If the rates of supply and withdrawal are correlated with the rate of advance of the propagating reaction zone through the reaction mass it is possible to maintain the interface more or less, or precisely, stationary.

In the reduction of UO₃ to UO₂ there may be formed the oxides U₃O₈ and U₄O₉. These, like UO₂, absorb the radiation, which is converted to heat, so that these oxides act generally in the same way as UO₂. These oxides can be used in place of UO₂ to form the initial interface with UO₃, i.e., these oxides can serve as the initiator. As the reaction proceeds in the presence of reducing gas the U₃O₈, U₄O₉ and UO₃ become converted to UO₂ which forms the interface with the UO₃ as the reaction proceeds.

By the method of the invention it is possible to obtain UO₂ with a stoichiometric ratio of O:U that is less than 2.04:1 and preferably less than 2.01:1. This UO₂ is particularly suitable for conversion to UF₄, so in a preferred aspect the invention includes the step of converting obtained UO₂ to UF₄, for instance by the reaction: ${{UO}_{2} + {4{HF}}}\overset{\quad {500{^\circ}\quad {C.}}\quad}{\rightarrow}{{UF}_{4} + {2H_{2}O}}$

or by treatment of UO₂ with dichlorodifluoromethane, at 500-600° C. (see, for example, Van Nostrand's Scientific Encyclopedia, 5th Edition, pg. 2264, and Advanced Inorganic Chemistry, Cotton, F. A., and Wilkinson, G., 5th Edition, pg. 1004) the disclosures of which are incorporated herein by reference.

Although the invention provides UO₂ with a low O:U ratio that is particularly suitable for conversion to UF₄, the UO₂ can be used for other purposes. The UO₂ can be sintered and compacted to form pellets, particularly ceramic pellets, to be used in fuel rods, for instance.

Depending on rate of reducing gas supplied and the incident microwave or radio-frequency power, the method of the invention can also yield UO₂ in which the O:U ratio is greater than or equal to 2.04. For some purposes UO₂ with a stoichiometric ratio of O:U that is 2.04:1 or greater is acceptable, and such UO₂ can be obtained by using less reducing gas, or by using a lower temperature in the reaction zone, say below 800° C., or both.

The heating is carried out by means of radio-frequency or microwave radiation. It is particularly preferred to use microwave radiation and at many places in the following description there is reference only to microwaves. It should be understood that radiofrequency radiation can be used, and reference to microwave radiation should be understood to extend to radiofrequency radiation unless the context requires otherwise. The electromagnetic energy which is preferred for the implementation of the method described, is in the range from 100 kHz to 10 GHz. Preferable industrial frequencies include 2450 MHz, 915 MHz (U.S.A. and Canada), 860 MHz (UK), and 40.68 MHz.

The container can be constructed of any material that will not adsorb microwaves and can withstand temperatures of the order of 1500° C. or higher. It is particularly preferred to use quartz. Other suitable materials include refractory materials with low dielectric loss, for example alumina, magnesia, zirconia and boron nitride. In a preferred embodiment, the container is enclosed in a microwave applicator, or it can act as a microwave applicator (in this case the container is made of a conducting material such as aluminum, stainless steel, etc.). In the case when the container confines the processed material inside a microwave or radio-frequency applicator, it is preferable to use thermal insulation between the container and the applicator walls to reduce the heat loss, increase the uniformity of the temperature, and to protect the applicator walls from excessive heating.

The relative disposition of the UO₃ and the UO₂ or other initiator material is not particularly limited, so long as they share an interface or common border region that upon irradiation will become the propagating reaction zone. This common border region can be substantially vertical or substantially horizontal, or at an incline. The UO₂ initiator or other initiator material can be in granular form. It is preferred that it have an average particle size in the range from about one micron to about one centimeter. The actual size of the particles does not matter as long as the initiator is in contact with the UO₃ feed material and provides a good coupling with the microwave energy. It is preferable that the interface between the initiator and processing UO₃ material be approximated by a plane surface (in other words, only a small fraction of the initiator particles project into the feed material. Large projections might cause spontaneous and non-uniform development and propagation of the reaction zone and are therefore to be avoided).

The reducing gas may be, for example, hydrogen, methane or other hydrocarbons, synthetic/natural gas, carbon monoxide, ammonia, or a mixture of any of these gases. The hydrocarbon in the reactor must be in the gaseous phase but, at the elevated temperature of the reaction bed (typically 700° to 1200° C.), higher hydrocarbons are gaseous and any may be used. Preferred hydrocarbons may be C₂₋₁₂ or higher. The aforementioned gases may also be in admixture with an inert carrier gas, for example nitrogen or argon. It is possible to preheat the reducing gas but this is not normally necessary. Even with a liquid hydrocarbon, sufficient hydrocarbon vapour can be picked up by passage of carrier gas through the liquid at room temperature. Normally cool reducing gas encounters hot UO₂ and is heated thereby, also cooling the UO₂, so that the process proceeds in an energy-efficient manner. It is particularly preferred to use hydrogen, either alone or in combination with an inert gas, for example nitrogen or argon. The gas may be at ambient pressure, or at elevated pressure. A preferred pressure range is from ambient up to about 50 psig.

A reactor according to the invention may be equipped with means for transporting the UO₃ feed-material to the reactor, and means for transporting the uranium dioxide product from the reactor, permitting the reactor to be used in a moving bed continuous or semi-continuous process. Examples of means for transporting the UO₃ feed-material and the uranium dioxide product are a powder feeder, a conveyor, a pneumatic transport system, and a gravity transport system.

Alternatively, the reactor may be used in a batch process.

The UO₃ feed-material can be in granular form. Suitable UO₃ is usually obtained by thermal decomposition of uranium salts such as uranyl nitrate, ammonium diuranate, etc. There is no preference with respect to the crystal modification of UO₃; it may be used in any of its crystal forms or in amorphous form. The preferred range for the average particle size is from about 30 μm to about 1cm, more preferably about 50 to about 300 μm. However larger or smaller particles may also be used, since the reaction zone will propagate regardless of the particle size provided that a sufficient amount of a reducing gas is supplied. Smaller particles may be processed, but a bed consisting of fine particles would have a high aerodynamic resistance with respect to the flow of reducing gas and is therefore less preferable.

Although it is preferred to have a dry UO₃ as a feed material, it may contain some moisture, for instance due to its hygroscopicity, without significantly influencing the performance of the microwave reduction process. Positive results have been obtained with UO₃ having a water content of 5% corresponding to the UO₃.0.8H₂O crystal hydrate. Water present in the UO₃ may influence transport properties of the UO₃, causing bridging and sticking to reactor walls and therefore may reduce overall performance. Steam is one of the products of the UO₃ reduction. It passes through the UO₃ bed with the flow of carrier gas and may be partly absorbed by UO₃. Typical UO₃ source powder contains less than one percent of nitrate. The nitrate is decomposed in the reaction zone giving rise to UO₃ and a mixture of gases (nitrogen dioxide and molecular oxygen). The UO₃ is reduced to UO₂ and the gases leave the reactor with or without being reduced by hydrogen or methane or other reducing gas. The requirements for the concentration levels of other impurities are determined by the required quality of the UO₂ product. In available UO₃, typical concentration levels of chlorine and sulfur are no more than about 100 ppm and typical concentrations of phosphorus and calcium are less than about 50 ppm and these concentration levels are acceptable and do not interfere significantly with the desired reaction.

As stated above, pure dry UO₃ can not be heated by microwave radiation, owing to its low dielectric loss. It is therefore necessary that reaction shall be initiated in some way. A preferred way is to use an amount of UO₂ which is heated by the microwave radiation. Heat from the microwaved UO₂ is conveyed to the UO₃ at the border between the UO₃ and UO₂ by conduction from the microwave-heated UO₂ and also by convection as the reducing gas passes through the heated UO₂ and into the UO₃. The UO₃ decomposes under the influence of heat and the reducing gas, and there is formed UO₂. The radiation is absorbed within the reaction zone, so the process is very energy efficient. As intermediate products of the decomposition of UO₃, which undergo further decomposition, there may be formed oxides of uranium intermediate between UO₃ and UO₂, for instance U₃O₈ and U₄O₉. These oxides will absorb microwave radiation and can therefore be used in place of UO₂ to generate the heat needed to initiate decomposition of the UO₃. As the reaction proceeds UO₂ is formed so that once a steady state is reached the interface is largely or entirely between UO₃ and UO₂.

As UO₃ is decomposed, the amount of UO₃ in the reactor is decreased and, correspondingly, the amount of UO₂ is increased. Thus, there is a tendency for the interface, or border region between UO₃ and UO₂ to move, progressing from the UO₂ into the UO₃, so that there is created the propagating reaction zone. If the microwave radiation is to be directed through the UO₃ towards the interface it is desirable that the UO₃ shall be substantially dry and substantially free from other material, for example impurities, that will absorb the radiation. The source UO₃ material is hygroscopic and may contain absorbed water and some impurities which may absorb the microwave energy, e.g. U₃O₈ or UO₂. It is desirable that the thickness of the layer of the UO₃ feed material above the interface, in the case when the feed contains microwave-absorbing impurities, be smaller than the penetration depth of microwave radiation in the feed material, so that the microwave radiation reaches the interface and produces heat within the reaction zone. The penetration depth in the UO₃ feed material is a function of composition of the feed material and decreases with increasing concentration in the UO₃ of impurities with high dielectric loss factor such as UO₂, U₃O₈ and other non-stoichlometric uranium oxides. As a result, no definite number can be provided for the thickness of the feed material. A typical range for the thickness of the UO₃ layer is from about 10 mm to about 0.5 m. Desirably the UO₃ material should not contain microwave-absorbing impurities at an amount greater than about 5%. This does depend upon the nature of the particular impurity, however. Applicant's experience shows that UO₃ containing more than 30 wt % of UO₂, or more than 60 wt % of U₃O₈ can be reduced to UO₂ by the process of the invention.

In general, UO₃ may be contaminated with other uranium oxides. The presence of such contaminants will reduce the depth of penetration of microwave energy in such materials as compared with pure UO₃. When using such contaminated UO₃, if the microwave energy is being passed through the contaminated UO₃ it is necessary to reduce the length of the path of the microwave energy in the contaminated UO₃, so that microwave energy is delivered to the interface between the UO₃ and UO₂.

Instead of or in addition to UO₃, other source materials may be used in the suggested process to produce UO₂. Such materials include uranium compounds that are not good microwave absorbers but may thermally decompose giving rise to UO₃ or U₃O₈. Examples of such materials include uranyl nitrate, UO₂(NO₃)₂, or uranyl nitrate hydrates, ammonium diuranate, (NH₄)₂U₂O₇, tetraammonium tricarbonatouranylate, (NH₄)₄UO₂(CO₃)₃, “yellow cake”, etc. Hence these materials can be used as precursors for in situ preparation of UO₃, which then undergoes further reaction. When these materials or their mixture used as a starting material, in addition to or instead of UO₃, are in close contact with the hot reaction zone, they are heated by thermal conductivity or by the hot reducing gas passing through the reaction zone and thermally decompose, presumably to UO₃. For example, uranyl nitrate decomposes to UO₃ at about 300° C. The UO₃ formed is reduced, preferably to UO₂, by the reaction mechanism described above. In the presence of a reducing gas, the uranium compounds may also be decomposed to the oxides other than UO₃ and further reduced to UO₂. The gas produced in the decomposition may contain ammonia, carbon dioxide and water, depending on the material used. The gas is removed through the UO₃ bed (or the bed of some of the above mentioned materials or their mixture) with the flow of reducing gas.

One embodiment of the invention is described in more detail. The components of the reaction system are placed inside a microwave/radiofrequency reactor in such a way that the UO₃ feed material and the UO₂ initiator and product form two contiguous layers and the microwave/radiofrequency energy is supplied from the side of the UO₃ layer (feed material input), and a reducing gas (typically, a hydrogen/nitrogen or hydrogen/methane mixture) is supplied from the side of the UO₂ product output. The incident electromagnetic radiation penetrates through the UO₃ layer with negligible attenuation and is absorbed initially by the UO₂ and subsequently also by the reduced oxides at the border between the UO₃ and UO₂ layers, releasing heat. The heat is transported to the adjacent UO₃ layer, by thermal conductivity and gas phase forced convection, and increases the temperature of the UO₃. It has been found that the temperature may rise to above 700° C. within a few millimeters of the interface between the layer of UO₂ and the layer of UO₃. As a result, this thin layer begins to absorb the microwave/radiofrequency radiation, its temperature increases further, and thermal decomposition of UO₃ takes place. The decomposition may take place in accordance with equation (2).

UO₃→⅓U₃O₈+⅙O₂.  (2)

In the flow of reducing gas the intermediate U₃O₈ is quickly reduced to UO₂ with an O/U ratio of less than 2.04, preferably 2.00 to 2.01. When hydrogen is used as reducing gas the reduction can be represented by equation (3).

⅓U₃O₈+⅔H₂→UO₂+⅔H₂O.  (3)

As demonstrated in the examples below, it is found that the amount of consumed hydrogen is less than the stoichiometric amount for the reduction of UO₃ to UO₂ by equation (1). It is assumed that this is a consequence of reaction in accordance with equation (3), and in some instances a 33% saving in hydrogen, has been achieved. In some cases, the savings are even greater, since the intermediate oxide has a stoichiometry of less than UO_(2.67) because of decomposition to U₄O₉ at a higher temperature. When the temperature of the reaction zone is sufficiently high (in a range of 700 to 1400° C.), the hydrogen may be completely consumed by the U₃O₈ (and/or U₄O₉). In the presence of methane or other reducing species in the gas phase, additional savings in hydrogen are obtained due to participation of these molecules in the reduction process. The high reduction rate of UO₃ by methane or other reducing gas that may act as hydrogen substitute is achieved due to the high temperature in the localized reaction zone under microwave/radlofrequency Irradiation.

The invention is further illustrated in the accompanying FIG. 1 showing, by way of example and in schematic form, one embodiment of the invention.

In a vessel 1 there is contained a layer 2 of UO₃, directly above a layer 4 of UO₂. Incident microwave radiation 6 travels through the layer 2 of UO₃ and enters an interfacial reaction zone 3 where it encounters the UO₂ and possibly other reduced oxides of uranium in layer 4. Hydrogen reducing gas 7 is passed through the layer of UO₂ and the layer of UO₃, countercurrent to the radiation. Fresh UO₃ is supplied as indicated at 5, and UO₂ is removed as indicated at 8. As reaction proceeds the reaction zone 3 propagates in an upward direction into the layer 2 of UO₃.

The invention is further illustrated in the following examples. It is to be understood that the examples in no way limit the scope of the invention.

EXAMPLES Example 1

A 300 g load of non-pulverized UO₃ powder having a moisture content of about 0.1% was placed in a quartz tube of 1″ diameter on the top of a layer of 25 g UO₂ supported on a porous alumina disk. The quartz tube was placed inside a vertical microwave cavity connected to a microwave generator capable of operating at a frequency of 2450 MHz, in such a way that in operation the microwave energy passed first through the UO₃ layer, before encountering the UO₃/UO₂ interface. A mixture of hydrogen and nitrogen gas was supplied to the bottom of the quartz tube and passed upwards through the tube. When the microwave generator was switched on at a 0.5 to 1 kW power level, the UO₂ heated up to a temperature of 900 to 1100° C. The gas coming out of the top end of the quartz tube contained water vapor as a product of the uranium oxide reduction. When the reaction was completed, all the UO₃ layer was converted to UO₂ with an O/U ratio in a range of 2.00 to 2.01. The UO₂ product had a uniform particle size distribution in the range from 0.1 to 0.3 mm, with high sphericity and high flowability that significantly exceeded the flowability of the UO₃ starting material. The flowability was measured using a Hall Flowmeter and the Standard Test Method for Flow Rate of Metal Powders, ASTM B 213-90, with a 50 g load of UO₂ and a calibrated orifice of 2.54 mm in diameter. The flow time for the microwave-reduced UO₂ was found to be 21 s, whereas for the starting UO₃, it was found to be 46 s. The sphericity was measured as an average ratio of the minimum to maximum particle diameters on magnified photo images of the UO₂ product powder. Most particles had a sphericity from 0.8 to 1.0 which explains the high flowability of the product. The O:U ratio was determined by a standard gravimetric technique (by determining the weight change in the sample under conditions when it is converted completely to U₃O₈). The UO₂ structure and O/U ratio were confirmed by X-ray diffraction (XRD) analysis.

The hydrogen consumption was measured with a calibrated mass flow controller and was found to be 62% of the stoichiometric amount necessary to reduce UO₃ by Equation (1). It is believed that the decrease in hydrogen consumption was due to thermal decomposition of UO₃ in accordance with equation (2) above.

Example 2

Two powder feeders were connected, one to the top end and one to the bottom end of the quartz reactor described in Example 1. The bottom part of the reactor was filled with UO₂. The top powder feeder supplied UO₃ powder (540 g total) into the reactor above the UO₂ material. The bottom feeder operated synchronously with the top feeder to remove UO₂ product from the reactor. The product coming out of the bottom feeder was collected in a container under inert (nitrogen) atmosphere, to avoid oxidation during cool down. The reducing hydrogen/nitrogen mixture was supplied from the bottom end, and the exhaust gas containing water vapors came out at the top end.

UO₃ reduction with hydrogen was initiated at a microwave power of 1 kW. The material collected in the container at the bottom of the reactor had a composition of UO₂ with an O/U ratio of 2.00 to 2.01, determined as described in Example 1. The amount of hydrogen consumed in the reduction was 67% of the stoichiometric value, again determined as described in Example 1.

Example 3

A 450 g load of non-pulverized UO₃ powder was placed in the reactor as described in Example 1. The bottom part of the reactor was filled with UO₂. A gas mixture of hydrogen and methane was supplied from the bottom of the quartz tube. The reduction was initiated at 0.5 to 1 kW microwave power level. The gas coming out of the top end contained water vapor and carbon oxides. When the reaction was completed, all the UO₃ layer was converted to UO₂ with an O/U ratio of 2.00 to 2.01. The hydrogen consumption was found to be 40% of the stoichiometric amount necessary to reduce UO₃ by Equation (1).

It is believed that the hydrogen consumption was lower than that required by Equation (1) because a fraction of the UO₃ was reduced by methane, and also because of decomposition of UO₃ according to Equation (2).

Example 4

The reactor set-up was as described in Example 2. The top container was filled with coarse non-pulverized UO₃ powder (0.3 mm average diameter) containing some pieces of the size of a few mm. The bottom part of the reactor was filled with UO₂. A gas mixture of hydrogen and natural gas was supplied from the bottom of the quartz tube. The reduction was initiated at 0.5 to 1 kW microwave power level. The gas coming out of the top end contained water vapor and carbon oxides. During the reduction, all the feed UO₃ material (540 g total) was converted to UO₂ with an O/U ratio of 2.00 to 2.01 and collected, under an inert atmosphere, in the bottom container. The hydrogen consumption was found to be 43% of the stoichiometric amount necessary to reduce UO₃ by Equation (1).

Example 5

The reactor set-up was as described in Example 4. The top container was filled with sieved UO₃ powder (0.1 mm average pi diameter). The bottom part of the reactor was filled with UO₂. A gas mixture of hydrogen and natural gas was supplied from the bottom of the quartz tube. The reduction was initiated at 0.5 to 1 kW microwave power level. The gas coming out of the top end contained water vapor and carbon oxides. During the reduction, all the feed UO₃ material (537 g total) was converted to UO₂ with an O/U ratio of 2.00 to 2.01 and collected in the bottom container. The hydrogen consumption was found to be 40% of the stoichiometric amount necessary to reduce UO₃ by Equation (1). 

What is claimed is:
 1. A method of reducing UO₃ which comprises subjecting UO₃ to heat generated by microwave or radiofrequency radiation wherein the radiation is supplied from a direction in which it encounters an interface between UO₃ and another oxide of uranium without having first passed through said other oxide of uranium.
 2. A method according to claim 1 wherein the UO₃ is subjected to a reducing gas, wherein UO₂ is produced.
 3. A method according to claim 2 wherein said other oxide of uranium is UO₂, U₃O₈, U₄O₉ or a mixture thereof.
 4. A method according to claim 2 wherein said other oxide of uranium is a nonstoichiometric oxide of uranium, or a mixture of nonstoichiometric oxides of uranium.
 5. A method according to claim 2 wherein said other oxide of uranium is UO₂.
 6. A method according to claim 2 wherein the reducing gas is selected from the group consisting of hydrogen, methane, a C₂₋₁₂ hydrocarbon, natural gas, carbon monoxide, ammonia mixtures thereof, and mixtures thereof with an inert gas.
 7. A method according to claim 2 wherein the reducing gas is hydrogen.
 8. A method according to claim 2 wherein the reducing gas is at ambient pressure.
 9. A method according to claim 2 wherein the reducing gas is at an elevated pressure of up to 50 psig.
 10. A method according to claim 2 wherein the UO₃ is heated to a temperature in excess of 700° C.
 11. A method according to claim 2 wherein the microwave or radiofrequency radiation is directed through the UO₃ to the said interface.
 12. A method according to claim 2 wherein the amount of radiation and the amount of reducing gas are selected so that the UO₂ product has a stoichiometric ratio O:U that is less than 2.04:1.
 13. A method according to claim 2 wherein the amount of radiation and the amount of reducing gas are selected so that the UO₂ product has a stoichiometric ratio O:U that is less than 2.01:1.
 14. A method according to claim 2 wherein the amount of radiation and the amount of reducing gas are selected so that the UO₂ product has a stoichiometric ratio O:U that is 2.04:1 or greater.
 15. A method according to claim 2 which comprises the further step of fabricating the obtained UO₂ into pellets for nuclear fuel.
 16. A method according to claim 12 which comprises the further step of converting the obtained UO₂ into UF₄.
 17. A method according to claim 2 wherein a precursor of UO₃ undergoes thermal decomposition to form UO₃ in situ. 